Apparatuses of a similar kind form the object of British Pat. No. 1 229 894 and Swedish Pat. No. 314 288. While in the refiner construction according to British Pat. No. 1 229 894 the discharge outlet for the treated material is positioned around the ends of the screw shafts remote from the inlet, the construction according to Swedish Pat. No. 314 288 provides for adjustability of the discharge opening between a stationary outlet end-wall of the housing and a longitudinally shiftable housing portion. The present invention is in the first place but not exclusively intended to be used in connection with constructions in which the outlet is not associated with the end-wall of the housing remote from the inlet but is disposed laterally in the stationary housing.